The present invention generally relates to methods of forming and aligning structures on patterned substrates. More specifically, the present invention relates to methods of molding and aligning glass, ceramic, and/or metal structures on patterned substrates for display applications, and to displays having barrier ribs molded and aligned using a stretchable mold.
Advancements in display technology, including the development of plasma display panels (PDPs) and plasma addressed liquid crystal (PALC) displays, have led to an interest in forming electrically-insulating ceramic barrier ribs on glass substrates. The ceramic barrier ribs separate cells in which an inert gas can be excited by an electric field applied between opposing electrodes. The gas discharge emits ultraviolet (uv) radiation within the cell. In the case of PDPs, the interior of the cell is coated with a phosphor which gives off red, green, or blue visible light when excited by uv radiation. The size of the cells determines the size of the picture elements (pixels) in the display. PDPs and PALC displays can be used, for example, as display screens in high definition television (HDTV) or other digital electronic displays.
Various methods have been used to fabricate ceramic barrier ribs for PDPs. One method is repeated screen printing. In this method, a screen is aligned on the substrate and used to print a thin layer of barrier rib material. The screen is removed and the material is hardened. Because the amount of material that can be printed with this technique is insufficient to create ribs having the desired height (typically about 100 xcexcm to 200 xcexcm), the screen is then realigned and a second layer of barrier rib material is printed on top of the first layer. The second layer is then hardened. Layers of rib material are repeatedly printed and hardened until the desired barrier height is achieved. The multiple alignment and hardening steps required with this method results in a long processing time and poor control of the overall barrier rib profile shape.
Another method involves masking and sandblasting. In this method, a substrate having electrodes is coated with the barrier rib material which is partially fired. A mask is then applied to the barrier material using conventional lithography techniques. The mask is applied on the areas between the electrodes. The substrate is then sandblasted to remove the barrier rib material exposed by the mask. Finally, the mask is removed and the barrier ribs are fired to completion. This method requires only one alignment step and can therefore be more accurate than the multiple screen printing method. However, because the area of the finished substrate covered by barrier ribs is small, most of the barrier rib material must be removed by sandblasting. This large amount of waste increases the production cost. In addition, because the barrier rib material often includes lead-based glass frit, environmentally-friendly disposal of the removed material is an issue. Also, while the positions of the ribs after sandblasting can be quite accurate, the overall shapes of the ribs, including the height-to-width aspect ratio, can be difficult to control.
Another process utilizes conventional photolithographic techniques to pattern the barrier rib material. In this technique, the barrier rib material includes a photosensitive resist. The barrier rib material is coated onto the substrate over the electrodes, often by laminating the rib material in the form of a tape onto the substrate. A mask is applied over the barrier rib material and the material is exposed by radiation. The mask is removed and the exposed areas of the material are developed. Barrier rib material can then be removed by washing to form the rib structures. This process can give high precision and accuracy. However, as with sandblasting, much material is wasted because the entire substrate is initially coated with the barrier rib material and the ribs are patterned by material removal.
Another process involves using a mold to fabricate barrier ribs. This can be done by direct molding on the substrate or by molding on a transfer sheet and then transferring the ribs to a substrate. Direct molding onto a substrate involves coating either the substrate or the mold with barrier rib material, pressing the mold against the substrate, hardening the material on the substrate, and removing tile mold. For example, Japanese Laid-Open Patent Application No. 9-134676 discloses using a metal or glass mold to shape barrier ribs from a glass or ceramic powder dispersed in a binder onto a glass substrate. Japanese Laid-Open Patent Application No. 9-147754 disclosed the same process where electrodes are transferred to the substrate simultaneously with the barrier ribs using a mold. After hardening the barrier rib material and removing the mold, the barrier ribs are fired to remove the binder.
European Patent Application EP 0 836 892 A2 describes printing a mixture of a glass or ceramic powder in a binder onto a transfer sheet. The material is printed using a roll or plate intaglio to form barrier rib shapes on the transfer sheet. A substrate is then pressed against the rib material on the transfer sheet to adhere the material to the substrate. After curing the rib material on the substrate, the ribs are fired. The transfer film can be removed before firing or burned away during firing.
While direct molding offers less wasted material than sandblasting or lithography and fewer alignment steps than screen printing, it poses challenges such as releasing the mold consistently and repeatedly from the barrier rib material and fabricating a separate mold for each unique display substrate. For example, slight adjustments in barrier rib pitch dimensions are desired to account for variations in shrinkage factors of glass substrates due to, for example, different lots or different suppliers.
If the barrier ribs are initially molded onto a transfer sheet, this method has the same disadvantages as direct molding. In addition, the transfer sheet with the rib material must be aligned with the electrodes on the substrate. This printing method may be used to print a pattern on a flexible film where the pattern on the film can subsequently be used as a mold for direct molding of barrier ribs. One difficulty, however, is that when the mold and rib material are pressed against the substrate to adhere the rib material to the substrate, the mold tends to elongate. This motion of the mold make precise alignment across the substrate very difficult. The method disclosed for solving this problem is to deposit a metal layer on the back of the mold to keep the mold from being able to elongate.
The present invention provides a method for forming and aligning microstructures on patterned substrates. Preferred embodiments of the present invention permit formation and alignment of microstructures on patterned substrates with high precision and accuracy over relatively large distances.
In a first aspect, the method of the present invention is a process for forming and aligning microstructures on a patterned substrate which proceeds by first placing a mixture comprising a curable material between a patterned substrate and a patterned surface of a mold. The patterned surface of the mold has a plurality of microstructures thereon. Microstructure as used in this application refers to indentations or protrusions in the surface of the mold. The mold is stretched to align a predetermined portion of the patterned surface of the mold with a correspondingly predetermined portion of the patterned substrate. The curable material between the mold and the substrate is cured to a rigid state adhered to the substrate. The mold is then removed, leaving hardened structures of the mixture aligned with the pattern of the substrate, the hardened structures replicating the microstructures of the patterned surface of the mold.
In another aspect, the present invention is a process for forming and aligning ceramic microstructures on a patterned substrate. A slurry is provided, the slurry being a mixture of a ceramic powder and a curable fugitive binder. The slurry is placed between a patterned glass substrate and a patterned surface of a mold, the patterned surface of the mold having a plurality of microstructures thereon. The mold is stretched to align a predetermined portion of the patterned surface of the mold with a correspondingly predetermined portion of the patterned substrate. The curable binder of the slurry is cured to harden the slurry and to adhere the slurry to the substrate. Then the mold is removed to leave green state microstructures of the slurry adhered to the substrate, the green state microstructures substantially replicating the microstructures of the patterned surface of the mold. The green state microstructures may be thermally processed to form substantially dense ceramic microstructures.
In another aspect, the present invention is a substrate element for use in an electronic display having microstructured barrier ribs molded and aligned on a patterned portion of a substrate. For example, the present invention provides a high definition television screen assembly including a plasma display panel. The plasma display panel includes a back glass substrate having a plurality of independently addressable electrodes forming a pattern and a plurality of ceramic microstructured barriers molded and aligned with the electrode pattern on the back substrate according to the process of the present invention. Phosphor powder is deposited between the ceramic barriers, and a front glass substrate having a plurality of electrodes is mounted with its electrodes orthogonally facing the electrodes of the back substrate. An inert gas is disposed between the front and back substrates.
In yet another aspect, the present invention provides an apparatus for molding and aligning ceramic microstructures on a patterned substrate. The apparatus stretches a stretchable mold having a microstructure thereon into close proximity with a patterned substrate, registers and aligns the microstructure of the mold with a predetermined portion of the patterned substrate, applies a slurry comprising a ceramic powder dispersed in a curable binder between the microstructure of the mold and the substrate, stretches the mold to align the microstructure of the mold with the predetermined portion of the patterned substrate, and cures the binder of the slurry between the substrate and the mold.